In a simple alternating current (AC) circuit, if the circuit is purely resistive, the current and voltage are in-phase, and true or active power is given by P=VI, in watts. If the loads are purely reactive, i.e., inductive or capacitive, then the voltage and current are ninety (90) degrees out of phase, and reactive power is given in Volt-Amperes-Reactive. In this case, there is no net transfer of energy to the load.
Practical loads have resistance, inductance, and capacitance, so both true and reactive power will flow to real loads. Even though the current associated with reactive power does no work at the load, it heats the wires, wasting energy.
In power metering applications, the most common way to measure active and reactive power is to convert current and voltage to digital words or streams with an analog to digital converter (ADC) and then apply a variety of processing to derive active and reactive power. For AC power, the current and voltage both have a fundamental frequency component at the line frequency ω. When doing the instantaneous power calculation, a multiplication is necessary between voltage and current channels. The result of the multiplication contains energy in DC (which is either active or reactive power depending on what quantities have been multiplied) and energy in the 2ω) component, where ω is the line frequency. Usually, this result is low pass filtered (LPF) to extract only the DC information but unless a large filter is applied, the remainder of the 2ω) is still large enough to create large jitters if a PWM (pulse width modulator) output is connected after the LPF.
Shown in FIG. 1 is an example conventional power meter calculation block for AC active or reactive power. The block 100 includes a pair of analog-to-digital converters 102a, 102b, for digitizing current I and voltage V, respectively. The outputs of the ADCs 102a, 102b are filtered by highpass filters (HPF) 104a, 104b, respectively, to remove offset components. Then the current I is multiplied at multiplier 110 with the voltage (switch 108 being closed) to obtain the instantaneous active power or (switch 108 being open), the voltage is applied to phase shifter 106 and then multiplied with the current to obtain the instantaneous reactive power.
The instantaneous powers include a DC component and a 2ω) component due to the multiplication. The instantaneous powers are then lowpass filtered using one or more LPFs 112, 114 to extract the desired quantity. Typically, the filtering is implemented as a simple IIR (infinite impulse response) first order LPF. As noted above, however, if a PWM 116 is applied at the output of the filtering, then jitter can still result from the 2ω) component.
Accordingly, there is a need for an improved and more stable output for power metering.